It's also very possible that he just doesn't love you. And never has. But aside from such speculation, you won't get anywhere without proper evidence of immigration fraud. I think that could jeopardize him. Having sex with other women outside the US is not such evidence however, I think. Or, if you think he is prone to felony, he might get himself in trouble and you can then report him to authorities if you want. Felony convictions can mess up a Green Card holder.
Anyways, good luck with your relationship!